This invention relates to plate-fin heat exchangers, and is more particularly directed to apparatus for belling and expanding hairpin tubes into a fin pack.
Plate-fin type heat exchangers are often employed in air conditioning systems and refrigeration systems. These plate-fin units are typically formed by lacing hairpin tubes or U-tubes into aligned holes in a stack of fin plates and tube sheets, with the U-bend sections extending out one side of one of the tube sheets and the open ends of the hairpin tubes extending out the other tube sheet. The walls of the tube, which are typically copper, are then expanded radially into contact with the metal of the fin collars and the tube sheets, which establishes both good thermal contact and mechanical support. The hairpin tube open ends are belled, either before or after tube expansion, and return bends are soldered or brazed into the belled ends to close the flow circuit of the unit.
While the hairpin tubes can be supported from the U-bend side during expansion, i.e., a process known as compression expansion, such a technique is not preferred because of the tendency to bend the tubes, and also because of an uncertainty in establishing an offset distance between the open ends of the tubes and the tube sheet. A compression expansion technique is described in U.S. Pat. No. 4,228,573.
A tension-expansion technique involves belling the hairpin tubes prior to expansion and then supporting the tubes by their belled ends while expander rods are driven into the two legs of each hairpin tube. The tubes can be belled directly against the associated tube sheet so that the tube sheet supports the hairpin tubes during expansion, or else the bells can be formed at an established standoff distance above the tube sheet. In the latter case the belled ends can be supported in a clamping jaw or similar device during expansion. One technique for belling and expanding hairpin tubes in a plate-fin heat exchanger is described in U.S. Pat. No. 4,584,765.
To date, there has been no equipment or apparatus proposed which permits tension expansion of the hairpin tubes of a plate-fin heat exchanger and in which belling and expanding are carried out at a single station. Also, there has not been proposed a hairpin tube expander which permits the belled ends of the hairpin tubes to be offset a finite distance from the associated tube sheet. No apparatus has been proposed that permits belling and expanding of hairpin tubes where a tube sheet flange extends over the open ends of the hairpin tubes.